Method of making gas diffusion electrodes for electrochemical cells with acid electrolytes

ABSTRACT

A gas diffusion electrode for electrochemical cells with acid electrolytes, comprises a hydrophobic layer of gas permeable electrically conductive material having up to 0.2 mm of thickness and having pores with a most frequent pore diameter of about 1.8 microns and a hydrophilic layer containing a catalyst which is closely bonded to the hydrophobic layer. The hydrophilic layer has pores with a most frequent pore diameter of 0.09 microns.

This is a division of application Ser. No. 749,663 filed Dec. 13, 1976.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to the construction of electrodes andin particular to a gas diffusion electrode for electrochemical cellswith acid electrolytes which comprises a hydrophobic layer of gaspermeable electrically conductive material and a hydrophilic layercontaining a catalyst closely bonded to the hydrophilic layer.

2. Description of the Prior Art

The invention relates particularly to a gas diffusion electrode forelectrochemical cells with acid electrolytes, preferably a cathode forair-operation. In electrochemical cells with acid electrolytes, air canbe used as a cheap oxidant, without the necessity for an elaboratepurification of the air, as it is necessary in alkaline cells. Thecorresponding air electrodes (cathodes) can be designed as gas diffusionelectrodes.

In the operation of these electrodes under air (20% oxygen) with normalclimate-related humidity, there is a considerable voltage drop, comparedto the operation with pure oxygen. The resulting power drop can not besufficiently reduced by artificial liquidification of the air suppliedto the electrodes. Besides, this requires an additional engineeringeffort. Furthermore, the cell voltage builds up to a stationary value inload variations with a relatively large time constant.

SUMMARY OF THE INVENTION

The invention is based on the problem of considerably reducing the powerdrop in the operation of the electrode under air, and in maintaining itindependent of the humidity of the air. Finally, the cell voltage is tobuild up to a stationary value in load variations with a relativelysmall time constant.

According to the invention a hydrophobic, gas-permeable and electricallyconductive layer of up to 0.2 mm thickness with a most frequent porediameter of about 1.8 microns is closely bonded with a hydrophilic layerof about 0.3 microns thickness containing the catalyst, which has a mostfrequent pore diameter of 0.08 microns.

The advantages achieved by the invention consist particularly in a lowpower loss, compared to pure oxygen operation, in a power independent ofthe humidity of the air, and in quick setting times of the loadpotentials.

Accordingly it is an object of the invention to provide a gas diffusionelectrode for electrochemical cells with acid electrolytes whichcomprises a hydrophobic layer of a gas permeable electrically conductivematerial having a thickness of up to 0.2 mm and having pores with a mostfrequent pore diameter of about 1.8 microns and which is closely bondedto a hydrophilic layer containing a catalyst and which has pores with amost frequent pore diameter of 0.08 microns.

A further object of the invention is to provide a gas diffusionelectrode which is simple in design, rugged in construction andeconomical to manufacture.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference should be had to the accompanying drawing and descriptivematter in which there is illustrated a preferred embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Drawings

FIG. 1 is a schematic representation of a construction of a cathodeelectrode constructed in accordance with the invention; and

FIG. 2 shows a curve of the variations of cell voltages over periods ofoperating time indicating the time slope of the cell voltage underdifferent conditions obtained with the use of the electrode.

GENERAL DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings in particular the invention embodied thereinin FIG. 1 comprises an electrode with a hydrophobic gas-conducting layer1 comprising hard coal and polytetrafluorethylene with a particle sizeof 10 to 25 microns and a most frequent pore diameter of about 1.8microns, and a hydrophilic working layer 2 consisting of activatedcarbon doped with Pt, with a particle size of 0.5 microns and a mostfrequent pore diameter of 0.08 microns. The thickness of layer 1 isabout 0.2 mm and that of layer 2 is about 0.3 mm.

The gas-conducting layer 1 is supplied with air and the working layer 2with an acid electrolyte 4. The phase boundary electrolyte/air isdesignated with 5, and the connecting zone with 6.

For the production of the gas-conducting layer 1, about 45 g of hardcoal powder with a particle size of about 10 to 25 microns are stirredwith about 50% by weight polytetrafluorethylene in the form of asuspension in 250 ml hexane. Subsequently the mixture is filtered offand washed out thoroughly with 100 ml hexane and 200 ml acetone to forma moist pulp which is pressed in a filter press mold to a compact solidbody (size of layer about 100 cm², thickness 5 mm). Skins of about 1 mmare cut from this solid body, and the skins are rolled down to about 0.4mm thickness. The selected grain fraction of the hard coal powderrepresents the optimum between the opposing parameters of electricconductivity and permeability, on the one hand, and hydrophobicbehavior, on the other hand.

For the production of the working layer 2 containing the catalyst about35 g activated carbon powder with a particle size of 0.5 microns and theapplied platinum catalyst, amounting to about 10% by weight, are stirredwith about 18% by weight polytetrafluorethylene and about 17% by weightpolyethylene in 200 ml hexane and filtered off. The pulp is pressed in afilter press mold to a compact solid body with a layer size of 100 cm²,for example, and a thickness of 20 mm, and from this body are cut skinsof about 1 mm thickness which are rolled down to a thickness of about0.6 mm.

Then the two pre-rolled layers are superposed and rolled together to afinal thickness of about 0.5 mm. The electrode thus formed is sinteredunder nitrogen for about 30 minutes at 165° C and under a pressure of100 p/cm².

Due to the good permeability of the hydrophobic gas-conducting layer, 1of 6.10⁻⁴ cm² /s and the low thickness of 0.2 mm, the nitrogen notreacted during the loading is rinsed loose from connecting zone 6 inwhich the oxygen is absorbed, and it arrives in the active centers atthe pore edges without much hindrance, due to the small pore diameter of0.08 mu m of the electrolyte-filled working layer 2. The low thickness0.3 mm of this layer 2 insures a rapid elimination of the reactionpartners H⁺ -ions and water.

As indicated in frame 1 in broken lines, an additional porous andhydrophilic layer 7 of up to 0.2 mm thickness can be rolled on workinglayer 2, which is non-condutive for electrons. This additional layer hasa most frequent pore diameter of about <5 μm; as a starting material forthis layer can be used Al₂ O₃ powder, BaSO₄ powder, etc. in connectionwith polyethylene or polystyrene. The layer acts as an electrolytestorage and as an electronically insulating separator to the cathode.This additional layer 7 eliminates the necessity of repumping theelectrolyte.

FIG. 2 shows the time slope of the cell voltage under oxygen and airoperation, with and without wetting of the gases. There is no potentialdifference between dry and humid air either under weak or under heavyload. The potential drop when changing from oxygen operation to airoperation is not more than 30 mV in an idle run under weak load, and notmore than 60 mV under heavy load.

As seen from the diagram, the cell voltage U_(H) is 1 volt at the time 0in oxygen operation, dry. It is assumed that a load of 10 mA qcm isapplied after 2.5 minutes, so that the cell voltage drops within about15 seconds to 0.85 V. It is also assumed that the operation is changedto air dry after 60 minutes under the same load of 10 mA/qcm, so thatthe cell voltage assumed within 15 seconds a value of 0.82 V; this valuedoes not change with humid air.

If we change again after 180 minutes to oxygen operation, dry, we obtainagain, within 15 seconds, a cell voltage of 0.85 V. When the load isincreased after 240 minutes to 100 mA/qcm, the cell voltage drops withinabout 15 seconds to 0.55 V and, after changing to air operation dry orhumid, the cell voltage drops after 300 minutes within 15 seconds to0.49 V. If the load is removed after 420 minutes, the cell voltage risesin air operation within about 15 seconds to about 0.98 V.

While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. A method of forming a gas diffusion electrode forelectrochemical cells with acid electrolytes comprising, mixing hardcoal powder with a most common particle size of about 10 to 25 micronswith polytetrafluorethylene or the like in a suspension medium to form afirst pulp, pressing the first pulp into a first body, cutting a skinfrom the first body, rolling the skin to a thickness of about up to 0.4mm to form a prerolled hydrophobic layer, mixing catalyst containingactivated carbon powder with a most common particle size of about 0.5microns with polytetrafluorethylene or the like in a suspension mediumto form a second pulp, pressing the second pulp into a second body,cutting skins from the second body, rolling the skins to a thickness ofabout 0.6 mm to form a prerolled hydrophilic layer, superposing theprerolled hydrophilic and hydrophobic layers, and sintering the layerstogether with heat and under pressure and in an inert atmosphere to formthe electrode with final thicknesses, of up to 0.2 mm for thehydrophobic layer and of about 0.3 mm for the hydrophilic layer, whereinthe most frequent pore diameter of the hydrophobic and hydrophiliclayers is 1.8 microns and 0.08 microns respectively.